docs: Provide parse-support-md

docs: Provide parse-support-md

This utility reads json format pandoc output, from parsing one or more
SUPPORT.md files, and generates an HTML table element containing the
principal version and feature information.

This is rather hairier than I anticipated when I started out; hence
the 400-odd-line Perl script.

Machinery to assemble the appropriate inputs for parse-support-md
will be in the next commit.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>
Acked-by: Lars Kurth <lars.kurth@citrix.com>
---
v2: New in this version of the series.
v3: Refactor to introduce RealSect
v3: Add [*] footnote to all applicable entries, not just the last